Scroll or screw conveyor centrifuge decanters are well known. Typically such apparatus comprises an elongated bowl mounted for rotation about the longitudinal axis with a helical screw conveyor, coaxially mounted within the bowl, adapted to rotate at a speed slightly different than the speed of rotation of the bowl; the bowl is tapered or trunco-conical at its solids discharge end. The screw conveyor is formed of one or more helically arranged blades, which may be mounted on a hub or spider and disposed to sweep the surface of the bowl of the apparatus while propelling the centrifugally separated solids toward the solids discharge port. An example of such equipment in which the blade is helically mounted on a hub is described in U.S. Pat. No. 3,764,062, incorporated herein by reference.
In the operation of a screw conveyor centrifuge, a solids-liquids feed is introduced into the bowl, where, due to centrifugal force effected by rotation of the bowl, the feed separates into its component parts with the heavier solids being moved outboard of the liquids and adjacent to the bowl's inner surface. Since the bowl and screw conveyor are rotated at controlled different speeds, solids sedimented against the bowl inner surface are conveyed by the conveyor's blade along the bowl's annular surface until discharged from one or more ports at the trunco-conical end of the bowl.
The present invention provides that in screw conveyor, centrifugal apparatus for the continuous separation of solids/liquids mixtures, the leading surface of the conveyor blade adjacent to its distal edge is provided with an arcuate surface of defined curvature and slope. Such arcuate surface is that portion of the blade adapted to contact the solids upon separation, at least within the trunco-conical portion of the bowl. The arcuate surface is concave in the direction of the solids discharge port and has a profile in a plane through the axis of rotation of the blade whereby (i) a line disposed in the plane and perpendicular to the bowl surface and a straight line tangent to the slope of the profile adjacent the distal edge define an angle .alpha. of between 10.degree. and 70.degree. subtended within the concavity (ii) the straight line tangent to the slope of the profile most distant from the distal edge (on the opposite side of the cavity) defines an angle .beta. between about 7.degree. and 90.degree. subtended within the concavity with a line disposed in the plane and perpendicular to the axis. The arcuate surface, in use, propells the separated solids toward the solids discharge port, while tumbling them, due partly to the curvature of the surface and partly to the rotation of the bowl and the blade at a speed differential.